Joint transform correlation is a process for optically comparing a test pattern (representative of the test object) with a reference pattern (representative of the reference object) to determine the existence of similarity between the test and reference objects. Extant on-axis correlators, an example of which is diagrammed in FIG. 1, that use transmission devices, such as second spatial light modulator 213 in the filter plane, require two different frequencies of coherent light to operate. A beam of .omega..sub.1 (write-beam) emanating from first laser 201 is needed to be transmitted through test pattern 227 and reference pattern 229 (together referred to as the input patterns). Even though in the figures the input patterns are depicted as separate from first spatial light modulator (SLM) 205, such depiction is for illustrative purposes only; but in practice, the input patterns reside on the first SLM. The first SLM can be something as simple as a transparency or slide containing the input pattern images or something as complex as a liquid crystal device wherein the pattern images are updated electronically. The first SLM encodes the images of the input patterns onto the beam of .omega..sub.1 as the beam passes through the first SLM. The encoded beam is then further transmitted to impinge on first focusing lens 207 where Fourier transform occurs and the resultant fringe image is projected from the first focusing lens toward second spatial light modulator 213. At this point, .omega..sub.1 is prevented from entering the rest of the system by use of filter 103; otherwise, .omega..sub.1 simply adds noise to the entire system and degrades the performance of the correlator. Then, with beamsplitter 101 in place as shown, a beam of .omega..sub.2 (read-beam) emanating from second laser 105 is allowed to propagate coaxially with .omega..sub.1 through second spatial light modulator 213 and through filter 103 onto second focusing lens 217 where Fourier transform once again occurs. Needless to say, then, the filter must be suitable to block the passage of .omega..sub.1 while allowing the passage of .omega..sub.2. At this juncture, if there is a match between the test and the reference patterns, a correlation pattern emerges. The correlation pattern consists of a triad of dots: a large dot in the middle with a smaller dot on either side of it. If no match exists between the input patterns, only the large dot in the middle appears. Camera 219 may be used to receive the triad or the single-dot image from the second focusing lens and television monitor 223 connected to the camera may be used to display the correlation triad or the single-dot.